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• #### Re:Can anyone explain... (Score:4, Interesting)

on Friday May 02, 2014 @10:00AM (#46898723) Homepage

You say it "appears to be travelling in the opposite direction," but does it? It never gets any closer or any further away. Both ships are travelling around a common center, but relative to each other, are they in fact (as far as time dilation is concerned) stationary?

So if both spaceships perceive that the other's clock is moving more slowly, what will they perceive if the spaceships come together to compare clocks?

You can simplify (or in the light my above thought, perhaps even "make valid") this particular question a bit by considering the twin paradox instead. The same thing happens - the twin who stays at home considers the travelling twin's clock to be slower, but so does the traveller. And it's the same on the home trip, too, even though the traveller is now moving towards the stay-at-home. What breaks the symettry of the situation is that only one of them undergoes acceleration at the start, turnaround, and end points.

If the traveller had stayed at Alpha Centauri and his lazy twin had ventured out to follow him at the same speed, their clocks would match.

I suspect that the same applies to the circling ships - the clocks will match if neither breaks symmetry, otherwise they won't. And if the symmetry break is done only at non-relativistic speeds - i.e., both twins slow to a stop, then one tootles over to the other at 10km/h - the clocks will practically match.

• #### Re: (Score:2)

What breaks my spelling of the word "symmetry" is another matter entirely.

• #### Re: (Score:2)

That point bears emphasising: in relativity, an accelerating object is distinct from the rest of the universe.

• #### Re: (Score:2)

It's Friday and I've had a long day. Why you gotta go putting conceptual bombs like that in my brain?

• #### Re: (Score:2)

Don't worry, general relativity makes the laws of the universe equivalent for all reference frames again. Of course, then you're dealing with general relativity.

• #### Re: (Score:2)

But those spaceships would be under constant acceleration. Their velocity is changing direction as they travel around the black hole.

• #### Re: (Score:2)

I don't remember anyone mentioning a black hole! In any case, they're both accelerating in an exactly opposite and symmetrical manner. There'd be no way to determine which of the two spaceships you were on if you were randomly teleported onto one of them (unless you had details of their positions at a particular time, so let's pretend you don't, or there are no useful navigational markers) so there can be no difference in their clocks if they meet up in a symmetrical way (and no practical difference if they

• #### Re:Can anyone explain... (Score:5, Informative)

on Friday May 02, 2014 @10:26AM (#46898951)

Well, there are a few things that would have to happen for them to compare clocks, and a key thing you're overlooking in your analysis:

1) For circular motion, the two ships would not have constant velocity in their _own_ reference frames - they're both accelerating towards the center (I'm assuming a flat space-time here for simplicity, but in GR things don't change much). Acceleration causes time dilation too!

2) For the ships to come together, they would have to maneouver. This will require further accelerations. Its during these that the other ship's clock will always appear to be moving faster.

What you've really got here is a reworking of the classical twin paradox - if one twin goes to Alpha Centauri (AC) and back, and the other stays on Earth, from _each_ perspective, the other one moves away then comes back. Yet the one who went to AC and back comes back younger - why? Well, what you're missing is that _at_ AC you have to slow down and then accelerate back towards Earth. This is the missing segment of the space-time picture, as the surfaces of simultaneity change during this acceleration.

I hope that clarifies things a bit.

• #### Any sign of Pierson Pupeteers? (Score:1)

by Anonymous Coward

What is the configuration of the star cluster?

• #### 2 million miles an hour ? (Score:3)

on Friday May 02, 2014 @09:55AM (#46898689)
Why not just use per second (I won't even harp on using 900 km.s-1 at that point it seems neigh useless) and compare to speed of light (that's about 0.3% of speed of light by the way) or other astronomical measurement.
• #### For the last time (Score:4, Funny)

on Friday May 02, 2014 @10:02AM (#46898735) Homepage
Don't come back!
• #### "Towards" at negative velocity? (Score:2)

A Globular Cluster Toward M87 with a Radial Velocity

Why don't they say "away from" at "(+)1000 km/s"? (if I've got that right; the somewhat hilarious "artist's impression" indicates, as does the headline, that the cluster is moving away from M87).

What's the significance of the negative velocity?

• #### Another Question (Score:4, Interesting)

on Friday May 02, 2014 @10:26AM (#46898947)
Another questions is what happens to the speeding cluster if is was flung out by a bigger galaxy. One would assume the the dark matter that originally present in the cluster would not take the same track. Without the supporting dark matter the radial velocities are too great for the outer stars of the cluster to continue orbiting the system. One would think that there should be trail of stars left behind. Could be a great way to investigate dark matter interaction with galaxies.
• #### Why wouldn't the dark matter simply go along? (Score:1)

Another questions is what happens to the speeding cluster if is was flung out by a bigger galaxy. One would assume the the dark matter that originally present in the cluster would not take the same track. Without the supporting dark matter the radial velocities are too great for the outer stars of the cluster to continue orbiting the system. One would think that there should be trail of stars left behind. Could be a great way to investigate dark matter interaction with galaxies.

Why would the dark matter stay when the rest of the cluster goes?

• #### Marrow (Score:2)

Come to think of it, this would be a great way to travel between galaxies. This would be an excellent Intergalactic Spacecraft!
• #### Re: (Score:1)

Let's call it an Intergalactic Bypass. Let's put a little sign next to it, saying "Don't Panic, next stop...the restaurant at the end of the universe."
• #### Re: (Score:2)

Not really. 2 million MPH is only like .003c, and the nearest galaxy to us is Andromeda, 2.5 million light-years away. You'd still be looking at a good 833 million years to get there.

Interstellar distances are huge. Intergalactic distances are brain-destroyingly huge.

• #### Re: (Score:2)

Not really. 2 million MPH is only like .003c, and the nearest galaxy to us is Andromeda, 2.5 million light-years away. You'd still be looking at a good 833 million years to get there.

True, at least you would be traveling in style, if you could live that long.

Interstellar distances are huge. Intergalactic distances are brain-destroyingly huge.

Absolutely. Distances withing the cluster would be interesting, considering the black holes compressed them together.

• #### Re: (Score:2)

Absolutely. Distances withing the cluster would be interesting, considering the black holes compressed them together.

Not so ; the suggestion is that the black hole interactions stripped the outer parts of the cluster off, leaving the most-tightly bound core region.

• #### Re: (Score:2)

Absolutely. Distances withing the cluster would be interesting, considering the black holes compressed them together.

Not so ; the suggestion is that the black hole interactions stripped the outer parts of the cluster off, leaving the most-tightly bound core region.

The distances between the stars within the most-tightly bound core region, ejected is what I was interested in.

I read the main article however the summary says (compressed into a space just a few dozen light-years apart) I must have missed where the suggestion was made about stars being striped off, but it's very interesting. You can only imagine what that would look like over millions of years. Thanks for pointing it out.

This is one of the reasons I love about being a geek and getting excited about the

• #### Towards Us (Score:5, Informative)

on Friday May 02, 2014 @10:29AM (#46898977)

The Virgo Cluster galaxy, M87, has ejected an entire star cluster, throwing it toward us at more than two million miles per hour.

I can imagine people getting alarmed at this, but they shouldn't. If it's truly directly towards us (unlikely), and never veers off course (unlikely), it would still take about 18.3 million years to reach us.

• #### Re: (Score:2)

This might be how advanced civilizations do space travel.

Oblig: I for one, welcome our new cluster-slinging galactic-traveling overlords!

• #### Re: (Score:1)

This might be how advanced civilizations do space travel.

No, it's how cheap civilizations do it. Advanced ones want a way to change their mind and turn back.

• #### Wan-Tu has been Busy - World at the End of Time (Score:1)

by Anonymous Coward

Frederick Phol
http://en.wikipedia.org/wiki/The_World_at_the_End_of_Time

An Interstellar Being that lives in the heart of a star "Flings" a cluster of Stars out of his Galaxy and far into the Future due to Time Dilation. On one planets that orbits one of the Stars lives a new colony of humans, cut off from Earth they come to understand something is wrong with their new home and the Universe recedes far into the past. Meanwhile one of the original colonists proceeds to "Time travel" himself due to cryonics sti

• #### They know something (Score:3)

<Todd_Lewis@unc.edu> on Friday May 02, 2014 @10:31AM (#46898995) Homepage
Sounds like M87's Puppeteers know something and are heading for higher ground.
• #### Hard to imagine cluster holding together... (Score:4, Interesting)

<thadbeier@gmail.com> on Friday May 02, 2014 @10:38AM (#46899051) Journal

For a three-body slingshot to work, the object would have to get pretty close to one or both of the black holes -- considerably closer than the size of a globular cluster. At that distance, the tidal forces around the black holes would rip the cluster apart. I just can't see this happening.

I suppose it's time to do some simulations :)

• #### Re: (Score:1)

For a three-body slingshot to work...

Yikes! Do not google that at work without safety settings

• #### Re: (Score:2)

For a three-body slingshot to work...

Yikes! Do not google that at work without safety settings

Damn, I wish I could mod that comment funny - and informative!

• #### Pedantic rant (Score:1, Flamebait)

a globular cluster of several thousand stars (compressed into a space just a few dozen light-years apart) is being thrown out of galaxy M87.

I always have issues with astronomical articles that say something *is* happening, especially when the observation is of a structure 53.4 million light years away. *Was* happening, sure. *Is* happening? Don't think so...

• #### Re: Pedantic rant (Score:1)

by Anonymous Coward

I always have an issue with pedantic neck beards that don't understand that from our pov, it is happening.

• #### In other words (Score:2)

it got cluster fscked

• #### The Cluster replies, (Score:4, Funny)

on Friday May 02, 2014 @11:44AM (#46899789)

"I've been thrown out of classier places than this!"

• #### Galactic Bullying. (Score:2)

"...The star cluster wandered too close to the pair...

"...The black holes then acted like a slingshot, flinging the cluster away at a tremendous speed."

For some reason I'm picturing two bullies who just caught the new freshman kid walking home from school...

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